This invention relates to acoustic surface wave devices in general and more particularly to a method for forming reflection gratings and overhanging ledges in the materials .alpha. quartz and LiNbO.sub.3 commonly used as substrates in such devices. Surface wave acoustic devices are gaining widespread use as filters, delay lines and the like. In particular, in frequency ranges between 10 mhz and 1 ghz, devices which are compact and provide numerous advantages over inductive capacitive type filters, and tuned electromagnetic wave guides are possible. This results directly from the fact that acoustic waves travel at a much slower speed than electromagnetic waves and thus, the size of a structure can be correspondingly smaller of the order of 10.sup.5.
When used in filtering applications these devices generally comprise a piezoelectric substrate on which are deposited two spaced transducers. The most common type of transducer used is what is known as the interdigital transducer wherein a plurality of fingers extend from a transducer pad on each side of the substrate and have overlapping portions. Electric fields created between the overlapping fingers of the transducer excite the piezoelectric material to generate the surface waves.
It is often desirable in surface wave devices to form reflection gratings which reflect the Rayleigh waves. For example, such ridges may be formed in the substrate at an angle of 45% with respect to the direction of propagation to cause a wave to make a 90.degree. turn. A pair of such gratings can be used to cause a reversal in the direction of the wave. Furthermore, it is also desirable for some applications to form over-hanging ledges in substrates to permit denser packing of wave guides than is possible with a conventional plate-like acoustic wave guide.
Heretofore, it has not been thought possible to etch geometrical structures into the materials .alpha. quartz and LiNbO.sub.3, both of which are widely used substrate materials, because of the unusual and severe chemical problems associated with these materials. Thus, where gratings or ledges were required, other types of materials had to be used. The primary problem encountered in trying to etch these materials was that of being able to apply a suitable mask or resist which would withstand the chemicals necessary for etching. The typical problem encountered was that of the mask or resist lifting off the surface of the substrate, thereby preventing selective etching.
Thus, it can be seen that there is a need for a controlled method of etching these desired materials, for use in acoustic wave guide devices.